Trench MOSFET and Method for Fabricating Same

ABSTRACT

According to an exemplary embodiment, a trench field-effect transistor (trench FET) includes a trench formed in a semiconductor substrate, the trench including a gate dielectric disposed therein. A source region is disposed adjacent the trench. The trench FET also has a gate electrode including a lower portion disposed in the trench and a proud portion extending laterally over the source region. A silicide source contact can extend vertically along a sidewall of the source region. Also, a portion of the gate dielectric can extend laterally over the semiconductor substrate. The trench FET can further include a silicide gate contact formed over the proud portion of the gate electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is generally in the field of transistors. More specifically, the present invention is in the field of trench-based field-effect transistors.

2. Background Art

Power semiconductor devices, such as trench field-effect transistors (trench FETs), are widely used in a variety of electronic devices and systems. Examples of such electronic devices and systems are power converters, such as DC to DC converters, in which vertically conducting trench type silicon FETs, for instance, may be implemented as power switches. In power converters, power losses within the power switches, as well as factors affecting switching speed, are becoming increasingly important. For example, for optimal performance, it is desirable to reduce overall gate charge Q_(g), gate resistance R_(g), and ON-resistance R_(dson) the power switches.

However, designing trench FETs to optimize performance for particular applications often involves tradeoffs, where improving one performance parameter degrades another. For example, reducing trench dimensions in a substrate can improve gate charge Q_(g) and ON-resistance R_(dson) at the expense of increased gate resistance R_(g). More particularly, reducing trench dimensions can also reduce the effective conductive area of a gate electrode in the trench, thereby increasing gate resistance R_(g). Thus, conventional trench FETs can be limited by trench dimensions in order to achieve acceptable overall performance. As such, it would be desirable to provide trench FETs which can have relatively improved gate resistance R_(g), while achieving other performance parameters.

Thus, there is a need for trench FETs that can overcome the drawbacks and deficiencies in the art and a method for fabricating the same.

SUMMARY OF THE INVENTION

A trench MOSFET and method for fabricating same, substantially as shown in and/or described in connection with at least one of the figures, and as set forth more completely in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a flowchart illustrating the steps taken to implement an embodiment of the present invention.

FIG. 2A illustrates a cross-sectional view, which includes a portion of a wafer processed according to an embodiment of the invention, corresponding to an initial step in the flowchart in FIG. 1.

FIG. 2B illustrates a cross-sectional view, which includes a portion of a wafer processed according to an embodiment of the invention, corresponding to an intermediate step in the flowchart in FIG. 1.

FIG. 2C illustrates a cross-sectional view, which includes a portion of a wafer processed according to an embodiment of the invention, corresponding to an intermediate step in the flowchart in FIG. 1.

FIG. 2D illustrates a cross-sectional view, which includes a portion of a wafer processed according to an embodiment of the invention, corresponding to an intermediate step in the flowchart in FIG. 1.

FIG. 2E illustrates a cross-sectional view, which includes a portion of a wafer processed according to an embodiment of the invention, corresponding to an intermediate step in the flowchart in FIG. 1.

FIG. 2F illustrates a cross-sectional view, which includes a portion of a wafer processed according to an embodiment of the invention, corresponding to a final step in the flowchart in FIG. 1.

FIG. 2G is a cross-sectional view showing trench field-effect transistors fabricated according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is directed to a trench MOSFET and method for fabricating the same. The following description contains specific information pertaining to the implementation of the present invention. One skilled in the art will recognize that the present invention may be implemented in a manner different from that specifically discussed in the present application. Moreover, some of the specific details of the invention are not discussed in order to not obscure the invention. The specific details not described in the present application are within the knowledge of a person of ordinary skill in the art.

The drawings in the present application and their accompanying detailed description are directed to merely exemplary embodiments of the invention. To maintain brevity, other embodiments of the invention, which use the principles of the present invention, are not specifically described in the present application and are not specifically illustrated by the present drawings.

FIG. 1 shows a flow chart illustrating a method according to an embodiment of the present invention. Certain details and features have been left out of flowchart 100 that are apparent to a person of ordinary skill in the art. For example, a step may consist of one or more substeps or may involve specialized equipment or materials, as known in the art. Steps 170 through 180 indicated in flowchart 100 are sufficient to describe one embodiment of the present invention; however, other embodiments of the invention may utilize steps different from those shown in flowchart 100. While steps 170 through 180 will be described with respect to fabricating an N channel device, it will be appreciated that the present invention is also applicable to P channel devices. It is noted that the processing steps shown in flowchart 100 are performed on a portion of processed wafer, which, prior to step 170, includes, among other things, a substrate, such as a silicon substrate and a TEOS layer formed over the substrate. The wafer may also be referred to simply as a wafer or a semiconductor die or simply a die in the present application.

Moreover, structures 270 through 280 in FIGS. 2A through 2F illustrate the result of performing steps 170 through 180 of flowchart 100, respectively. For example, structure 270 shows a semiconductor structure after processing step 170, structure 272 shows structure 270 after the processing of step 172, structure 274 shows structure 272 after the processing of step 174, and so forth.

Referring now to FIG. 2A, structure 270 of FIG. 2A shows a structure including a substrate, after completion of step 170 of flowchart 100 in FIG. 1. Structure 270 includes substrate 202, which can be, for example, an N type silicon substrate, and TEOS material 204 a, 204 b, and 204 c formed over substrate 202.

As shown in FIG. 2A, structure 270 further includes trenches 206 a and 206 b and respective openings 208 a and 208 b formed over trenches 206 a and 206 b. In structure 270, opening 208 a is formed between TEOS material 204 a and 204 b and opening 208 b is formed between TEOS material 204 b and 204 c. Trenches 206 a and 206 b and openings 208 a and 208 b can be formed, for example, by depositing a TEOS layer over a substrate. Photoresist can be deposited and patterned over the TEOS layer and openings can be formed in the TEOS layer (not shown in FIG. 2A). Thus, the TEOS layer can be used as a hard mask to form trenches 206 a and 206 b in substrate 202.

According to one embodiment, thermal oxide layers are grown in each trench 206 a and 206 b. Subsequently, a wet etch can be performed to remove the thermal oxide layers and to laterally extend the openings in the TEOS layer to widths 210 a and 210 b, thereby forming respective openings 208 a and 208 b, which are notably wider than respective trenches 206 a and 206 b. More particularly, because the etch rate of the thermal oxide layers is lower than the etch rate of the TEOS layer, the openings in the TEOS layer will etch at a faster rate than the thermal oxide layers during the wet etch. The wet etch can include an over etch, which can further extend the openings in the TEOS layer. Thus, gate dielectrics 212 a and 212 b can be formed in respective trenches 206 a and 206 b, each including respective portions 214 a and 214 b extending laterally in respective openings 208 a and 208 b over substrate 202. Gate dielectrics 212 a and 212 b can comprise, for example, silicon oxide (SiO2) formed by thermal oxidation. The result of step 170 of flowchart 100 is illustrated by structure 270 in FIG. 2A.

Referring to step 172 in FIG. 1 and structure 272 in FIG. 2B, at step 172 of flowchart 100, gate electrodes 216 a and 216 b are formed in respective trenches 206 a and 206 b and openings 208 a and 208 b. In structure 272, gate electrode 216 a includes lower portion 216 a 1 formed in substrate 202 and proud portion 216 a 2 formed in opening 208 a. Similarly, gate electrode 216 b includes lower portion 216 b 1 formed in substrate 202 and proud portion 216 b 2 formed in opening 208 b. Thus, proud portions 216 a 2 and 216 b 2 have respective widths 211 a and 211 b, which are greater than the widths of respective trenches 206 a and 206 b.

Gate electrodes 216 a and 216 b can be formed, for example, by depositing electrode material, such as, polysilicon into trenches 206 a and 206 b and openings 208 a and 208 b, and etching back the deposited polysilicon. In a specific example, the polysilicon can be etched back such that proud portions 216 a 2 and 216 b 2 each have a thickness greater than approximately 3000 Angstroms. The polysilicon can be highly doped and in some embodiments can be doped in-situ while in other embodiments it can be doped after being deposited. For example, for an N channel transistor, the polysilicon can be N++ in-situ doped polysilicon. The result of step 172 of flowchart 100 is illustrated by structure 272 in FIG. 2B.

Referring now to step 174 in FIG. 1 and structure 274 in FIG. 2C, at step 174 of flowchart 100, TEOS material 204 a, 204 b, and 204 c is removed using, for example, a TEOS etch-back. Notably, in removing TEOS material 204 a, 204 b, and 204 c, gate dielectrics 212 a and 212 b are substantially maintained. More particularly, because proud portions 216 a 2 and 216 b 2 have respective widths 211 a and 211 b, which are greater than the uppermost width of respective trenches 206 a and 206 b, proud portions 216 a 2 and 216 b 2 can protect respective gate dielectrics 212 a and 212 b during removal of TEOS material 204 a, 204 b, and 204 c. In one particular example, the uppermost width of trenches 206 a and 206 b can be approximately 0.2 to 0.3 microns and widths 211 a and 211 b of respective proud portions 216 a 2 and 216 b 2 can be approximately 0.3-0.4 microns.

As shown in FIG. 2C, channel regions 218 and source regions 220 are formed in structure 274. Channel regions 218 and source regions 220 can be formed, for example, by dopant implantation into substrate 202. For an N channel transistor, channel regions 218 can comprise P type regions and source regions 220 can comprise highly doped N type regions. Channel regions 218 are shown formed adjacent respective trenches 206 a and 206 b and below respective source regions 220. It is noted that, in the present example, widths 211 a and 211 b of respective proud portions 216 a 2 and 216 b 2 can be selected such that source regions 220 can substantially form under portions 214 a and 214 b of respective gate dielectrics 212 a and 212 b using dopant implantation.

Furthermore, in the present example, because channel regions 218 and source regions 220 are formed after trenches 206 a and 206 b, gate dielectrics 212 a and 212 b, and gate electrodes 216 a and 216 b, channel regions 218 and source regions 220 are not exposed to related process temperatures in forming those features and thus can be formed using more controlled temperatures if desired. However, it is reiterated that other embodiments of the invention may utilize steps different from those shown in flowchart 100. The result of step 174 of flowchart 100 is illustrated by structure 274 in FIG. 2C.

Now referring to step 176 in FIG. 1 and structure 274 in FIG. 2D, at step 176 of flowchart 100, spacer material 222 is formed over substrate 202. For example, spacer material 222 can be formed by conformally depositing silicon oxide (SiO2) over substrate 202. The result of step 176 of flowchart 100 is illustrated by structure 276 in FIG. 2D.

Referring to step 178 in FIG. 1 and structure 278 in FIG. 2E, at step 278 of flowchart 100, spacer material 222 is etched-back to form spacers 222 a and 222 b and to expose gate electrodes 216 a and 216 b and source regions 220. As shown in FIG. 2E, proud portions 216 a 2 and 216 b 2 of gate electrodes 216 a and 216 b are exposed. Also shown in FIG. 2E, spacers 222 a are formed adjacent respective sidewalls of proud portion 216 a 2 and spacers 222 b are formed adjacent respective sidewalls of proud portion 216 b 2.

Also in step 178, source regions 220 are etched to form source regions 220 a and 220 b and to expose channel regions 218, which, in the present example, can be accomplished using a self-aligned process with spacers 222 a and 222 b. As shown in FIG. 2E, source regions 220 a are adjacent respective sidewalls of trench 206 a and source regions 220 b are adjacent respective sidewalls of trench 206 b. Source regions 220 a and 220 b are further shown situated below respective proud portions 216 a 2 and 216 b 2 of gate electrodes 216 a and 216 b and above respective channel regions 218.

Also in step 278, contact regions 224 a, 224 b, and 224 c can be formed over respective channel regions 218. In one embodiment contact regions 224 a, 224 b, and 224 c can comprise highly doped P type regions formed, for example, using dopant implantation into channel regions 218. The result of step 178 of flowchart 100 is illustrated by structure 278 in FIG. 2E.

Referring to step 180 in FIG. 1 and structure 280 in FIG. 2F, at step 180 of flowchart 100, silicide gate contacts 228 a and 228 b are formed over respective gate electrodes 216 a and 216 b and silicide source contacts 226 a, 226 b, and 226 c are formed over respective channel regions 218. Silicide source contacts 226 a, 226 b, and 226 c and silicide gate contacts 228 a and 228 b can be formed, for example, by depositing a metal over substrate 202, annealing the metal, and removing unreacted material. In one embodiment, for example, the metal can comprise titanium and silicide source contacts 226 a, 226 b, and 226 c and silicide gate contacts 228 a and 228 b can comprise titanium silicide, however, other metals can be used to form other silicides.

In the embodiment shown in FIG. 2F, silicide source contacts 226 a, 226 b, and 226 c extend laterally on respective contact regions 224 a, 224 b, and 224 c. Also shown in FIG. 2F, silicide source contact 226 a extends vertically on a sidewall of source region 220 a, silicide source contact 226 b extends vertically on a respective sidewall of source regions 220 a and 220 b, and silicide source contact 226 c extends vertically on a sidewall of source region 220 c.

Also in structure 280, silicide gate contact 228 a is formed on proud portion 216 a 2 of gate electrode 216 a and silicide gate contact 228 b is formed on proud portion 216 b 2 of gate electrode 216 b. The result of step 180 of flowchart 100 is illustrated by structure 280 in FIG. 2F.

Additional steps can be performed on structure 280 to form structure 290 including trench FETs 240 a and 240 b as shown in FIG. 2G. In some embodiments a dielectric layer comprising, for example, SiO2, can be conformally deposited over structure 280 and a photomask can be used to etch the dielectric layer to form dielectric caps 230 a and 230 b. In FIG. 2G dielectric caps 230 a and 230 b are formed over respective proud portions 216 a 2 and 216 b 2 of gate electrodes 216 a and 216 b. Subsequently, source metal 232 can be deposited over substrate 202, where dielectric caps 230 a and 230 b insulate respective gate electrodes 216 a and 216 b from source metal 232.

As shown in FIG. 2G, source metal 232 contacts source regions 220 a and 220 b through silicide source contacts 226 a, 226 b, and 226 c. In the embodiment shown, each silicide source contact 226 a, 226 b, and 226 c extends vertically on a sidewall of source region 220 a and/or 220 b, and laterally on respective contact regions 224 a, 224 b, and 224 c. Thus, as shown in FIG. 2G, source metal 232 can contact source regions 220 a and 220 b in embodiments where, for example, dielectric caps 230 a and 230 b extend laterally over contact regions 224 a, 224 b, and 224 c.

In transistors 240 a and 240 b, silicide gate contacts 228 a and 228 b provide a low resistance path for a signal from a gate contact (not shown in the Figures). Furthermore, silicide-gate contacts 228 a and 228 b extend along the length of respective gate electrodes 216 a and 216 b and are coupled to the gate contact (not shown in the Figures). In one specific example, the cross-section shown in FIG. 2G can be remote from the location at which the gate contact is coupled to silicide gate contacts 228 a and 228 b, while silicide gate contacts 228 a and 228 b provide a low resistance signal path along the length of gate electrodes 216 a and 216 b to the cross-section. Thus, silicide gate contacts 228 a and 228 b can reduce sheet resistance in transistors 240 a and 240 b.

Furthermore, as discussed above, the invention can provide for, for example, trench FET 240 a including gate electrode 216 a having lower portion 216 a 1 formed in substrate 202 and proud portion 216 a 2 formed over lower portion 216 a 1. As shown in FIG. 2G, proud portion 216 a 2 is situated above source regions 220 a. Thus, proud region 216 a 2 can increase the effective conductive area of gate electrode 216 a. Furthermore, proud region 216 a 2 can have a width 211 a greater than lower portion 216 a 2, which can further increase the effective conductive area of gate electrode 216 a. As such, proud portion 216 a 2 can significantly reduce gate resistance R_(g), even when dimensions of trench 206 a are maintained.

From the above description of the invention it is manifest that various techniques can be used for implementing the concepts of the present invention without departing from its scope. Moreover, while the invention has been described with specific reference to certain embodiments, a person of ordinary skill in the art would appreciate that changes can be made in form and detail without departing from the spirit and the scope of the invention. Thus, the described embodiments are to be considered in all respects as illustrative and not restrictive. It should also be understood that the invention is not limited to the particular embodiments described herein but is capable of many rearrangements, modifications, and substitutions without departing from the scope of the invention. 

1. A trench field-effect transistor (trench FET) comprising: a trench formed in a semiconductor substrate, said trench including a gate dielectric disposed therein; a source region disposed adjacent said trench; a gate electrode including a lower portion disposed in said trench and a proud portion extending laterally over said source region.
 2. The trench FET of claim 1 further comprising a silicide gate contact formed over said proud portion of said gate electrode.
 3. The trench FET of claim 1, wherein a portion of said gate dielectric extends laterally over said semiconductor substrate.
 4. The trench FET of claim 1 further comprising dielectric spacers formed adjacent respective sidewalls of said proud portion of said gate electrode.
 5. The trench FET of claim 1 further comprising a dielectric cap formed over said proud portion of said gate electrode.
 6. The trench FET of claim 1, wherein a silicide source contact extends vertically along a sidewall of said source region.
 7. The trench FET of claim 6 further comprising a channel region disposed adjacent said trench below said source region.
 8. The trench FET of claim 7, wherein said silicide source contact extends laterally over said channel region.
 9. A method for fabricating a trench field-effect transistor (trench FET) comprising: forming a trench in a semiconductor substrate, said trench including a gate dielectric disposed therein; forming a source region disposed adjacent said trench; forming a gate electrode including a lower portion disposed in said trench and a proud portion extending laterally over said source region.
 10. The method of claim 9 further comprising forming a silicide gate contact over said proud portion of said gate electrode.
 11. The method of claim 9, wherein a portion of said gate dielectric extends laterally over said semiconductor substrate.
 12. The method of claim 9 further comprising forming dielectric spacers adjacent respective sidewalls of said proud portion of said gate electrode.
 13. The method of claim 9 further comprising forming a dielectric cap over said proud portion of said gate electrode.
 14. The method of claim 9 further comprising forming a silicide source contact extending vertically along a sidewall of said source region.
 15. The method of claim 15, wherein a channel region is disposed adjacent said trench below said source region.
 16. The method of claim 15, wherein said silicide source contact extends laterally over said channel region.
 17. A trench field-effect transistor (trench FET) comprising: a trench formed in a semiconductor substrate, said trench including a gate dielectric disposed therein; a source region disposed adjacent said trench; a gate electrode including a lower portion disposed in said trench and a proud portion extending above said source region, a silicide gate contact formed on said gate electrode.
 18. The trench FET of claim 17, wherein said proud portion extends laterally over said source region.
 19. The trench FET of claim 18, wherein a portion of said gate dielectric extends laterally over said semiconductor substrate.
 20. The trench FET of claim 17 further comprising dielectric spacers formed adjacent respective sidewalls of said proud portion of said gate electrode. 